AUSTIN — As soon as the lights fell for the Saturday premiere of the HBO documentary “Running with Beto,” former congressman Beto O’Rourke sneaked into the darkened movie theater with his wife and daughter.

The crowd spotted him and cheered. A photographer followed O’Rourke to his seat, the camera flash lighting the potential presidential candidate’s face. A woman in the third row whispered: “He’s like a movie star.”

In less than a year, O’Rourke has been transformed from a relatively unknown congressman from El Paso, running a long-shot campaign in Texas against Republican Sen. Ted Cruz, to a political celebrity who is regularly mobbed by adoring fans and urged by strangers to run for president — something that those close to him say he plans to do.

But celebrity exacts a cost, one that the documentary showed was borne by O’Rourke’s three young children. For 90 minutes on Saturday, O’Rourke was reminded in intimate detail just how difficult his last campaign was — and how grueling an even higher profile race may be — for his children.

O’Rourke gave the documentary crew full access to his family and several campaign staffers for more than a year, allowing them to gather 700 hours of footage. O’Rourke said he trusted the filmmakers to be respectful of his family and that the team was “genuinely interested in telling our story, we could feel that.”

The result was a glimpse at a wrenching reality rarely seen in the sanitized, smiling images usually put forth by candidates.

People gather outside the Paramount Theatre for the documentary on Beto O'Rourke, “Running with Beto” at the South by Southwest film festival. (Matt McClain/The Washington Post)

In one scene, O’Rourke’s youngest son, Henry, hid behind the couch and left a voice mail for his dad. In another, O’Rourke’s wife explained that the children started writing old-school letters to their father instead of video-chatting with him because “after they hung up on the phone . . . they were in tears and really upset.”

The O’Rourke children recounted watching two heavily armed gun-rights activists confront their father at a gun-control march. And on the night their father lost the election, the children discussed how it made them sad to watch others cry.

“I’m ready for it to be over,” his oldest son Ulysses, then 11, said late in the campaign when both of his parents were away from home several days a week.

Ulysses wasn’t the only one.

“I’m having a super hard time right now,” O’Rourke said at one point, driving himself to a campaign event. “To have like the Wall Street Journal reporter asking me 50 questions in an hour to then, right away, sit down in front of the NPR reporter and dance for a little while in front of him. And then, don’t eat, get up and go into this town hall and try to be genuine and direct with people. There’s just no time for your brain to relax and unclench and it . . .” He closed the thought with a vulgarity.

Beto O'Rourke walks on stage at the Paramount Theatre after the documentary was shown. (Matt McClain/The Washington Post)

The documentary — which was co-produced by Crooked Media, a firm founded by former Obama staffers — emotionally captures the Beto-mania that swept Texas last year and profiles three volunteers who deeply believed in O’Rourke. But it also presents a less-than-fawning look at O’Rourke — a candidate who got nervous before speaking to huge crowds or debating Cruz, who lashed out at his overworked employees, who acknowledged on election night that he had often been a “giant” jerk on the campaign trail, who had a complicated relationship with his own father and nonetheless thrust his young children into the spotlight.

Amy O’Rourke said in the film that her husband first pitched the idea of running for Congress when she was pregnant with their youngest.

“I was just crying and crying because I couldn’t understand why he would almost like sacrifice our family to run for that level of office,” she said. “Because everything that I knew about life on the Hill was just kind of dirty and slimy and people changed once they got there.”

Two years later, she had a change of heart. O’Rourke was elected to the House of Representatives three times, then was motivated by the election of President Trump to run for Senate — a decision that Amy O’Rourke said was “easy” to make.

Beto and Amy O’Rourke attended the premiere at the South by Southwest festival with their daughter — their two young sons opted not to come — and they answered a few questions from the audience afterward.

When asked if he planned to run for president, O’Rourke dodged, turning the conversation to several candidates for local offices. (He later told reporters that he wants to “do it the right way” and “on the timeline that works for my family and for the country.”) One questioner, a mother of three, thanked O’Rourke’s wife and daughter for “sharing your daddy with us” — the only allusion by the crowd to the costs incurred.

The friendly audience for the premiere was packed with former campaign staffers and volunteers who heartily laughed at every laugh line, cheered along with the cheering crowds in the documentary and relived the campaign, sometimes openly weeping. Later, as they left the theater, they provided gushing reviews. In interviews, some of O’Rourke’s supporters said they are ready to volunteer on a presidential campaign for him, while others said that as much as they love him, they don’t think he’s ready to run for the White House.

As the on-screen O’Rourke defended the right of football players to kneel in protest of inequality — a moment captured in a viral video that catapulted him into national prominence — his former campaign manager, Jody Casey, sat in the audience and applauded.

O’Rourke had a toothy smile frozen on his face for most of the documentary, although it tightened into a wince as he watched himself repeatedly criticize his road manager, Cynthia Cano, for not budgeting enough time in his schedule for media interviews, for not keeping him on schedule, for not properly preparing him for campaign events. Sometimes during these scenes, O’Rourke leaned across the aisle toward Cano and other staffers and appeared to whisper to them. As the documentary quickly rattled through the highlights of the final days of the campaign — O’Rourke climbing atop a vehicle in a parking lot to address supporters, meeting first-time voters, voting with his family, walking past a polling location as it closed — O’Rourke slumped toward his wife.

The cameras continued to follow O’Rourke as he prepared to give a concession speech and huddled backstage with his family and top staffers, telling them how much he loved them and marveling at how hard they had worked.

“You all never allowed my shortcomings to get in the way of running the best campaign this state has ever seen,” he told them.

The documentary ended and the audience applauded. O’Rourke, his family and his aides filed out of the darkened theater to prepare for a question-and-answer session. The credits rolled as Tom Petty’s “Runnin’ Down a Dream” blared. In the darkness, someone called out: “2020!”